1. Field of the Invention
The present invention relates to a seal, and more specifically to a bellows seal having a balanced, de-coupled seal ring and seal ring shell.
2. Discussion of the Related Art
Pumps, especially those in refineries and chemical plants, often handle difficult-to-seal liquids, including propane, butane, and other unstable, combustible, or toxic liquids. These liquids can cause short seal life and undesirable product leakage, prompting the passage of state health and safety pump emission regulations.
FIG. 5 shows a conventional bellows seal. A stationary assembly of the seal includes a stationary seal ring 200 fitted against a stationary seal ring shell 202 using a press fitting or a thermal shrink fitting technique, and a bellows 206 attached to a side of seal ring shell 202 opposite seal ring 200. Press fitting or thermal shrink fitting provides a very tight interference fit between stationary seal ring 200 and seal ring shell 202, wherein stationary seal ring 200 is radially and axially fixed to seal ring shell 202. In press fitting, mating parts, on which the outer dimension of the interior member is the same as or slightly greater than the interior dimension of the exterior member, are forced together. In shrink fitting, the parts are joined by contracting (shrinking) the interior part by cooling and inserting the interior part into the exterior part. Subsequent expansion of the interior part by its return to ambient temperature ensures a tight fit. Alternatively, the parts are joined by expanding the exterior part by heating and inserting the interior part into the exterior part. Subsequent contraction of the exterior part by its return to ambient temperature ensures a tight fit.
The interference fit between stationary seal ring 200 and stationary seal ring shell 202, acts as a secondary static seal prohibiting sealed process fluid from leaking between stationary seal ring 200 and stationary seal ring shell 202. Also, the contact friction between stationary seal ring 200 and stationary seal ring shell 202 caused by the interference fit prohibits relative circumferential movement of stationary seal ring 200 with respect to stationary seal ring shell 202. However, in order for this to occur, the friction between stationary seal ring 200 and stationary seal ring shell 202 must be greater than the rubbing friction created between the seal ring face and its mating rotating seal ring 204. Further, the interference fit between stationary seal ring 200 and stationary seal ring shell 202 prevents relative axial movement of stationary seal ring 200 with respect to stationary seal ring shell 202. To prohibit axial movement, however, the friction force between stationary seal ring 200 and stationary seal ring shell 202 must be greater than any axial forces exerted on stationary seal ring 200 at radially-extending surfaces of stationary seal ring 200 that are exposed to the hydrostatic pressure of the process fluid.
The interference fit between stationary seal ring 200 and stationary seal ring shell 202 also produces unwanted stresses in stationary seal ring 200 that cause seal ring face distortion, i.e., the seal ring face is twisted or bent. In bellows seal applications, however, it is preferred that the stationary seal ring be free from such face distortions. For example, in pump applications operating under normal pressure conditions, it is necessary for the seal ring to have a flat face. If a facially distorted seal ring is used in such applications, then the seal ring experiences unusual wear, shortening the life of the seal ring.